4) would be your correct answer
Isotopes are two atoms with the same number of protons, but a different number of neutrons. Because they have the same number of proton they are atoms of the same element, but with different masses.
so, the answer is c.
i hope this will help you
Cl2(g) -------> Cl-(aq) + ClO-(aq)
2e- + Cl2(g) -------> 2Cl-(aq) [reduction]
4OH-(aq) + Cl2(g) -----------> 2ClO-(aq) + 2H2O(l) + 2e- [oxidation]
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2OH-(aq) + Cl2(g) --------> Cl-(aq) + ClO-(aq) + H2O(l)
Earth spins at 23.5
A day is one rotation